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Optical MineralogyOptical Mineralogy
WS 2007/2008WS 2007/2008
Last week….Last week….
Length fast and length slow
Twinning
Zoning
Exsolution
Undulose extinction
How the diagrams in Tröger relate to the optical properties of minerals
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Optical characterOptical character
Long dimension of mineral is parallel to the slow ray (n , nZ) =
LENGTH SLOW
Long dimension of mineral is perpendicular to the slow ray (n ,
nZ) =
LENGTH FAST
sillimanite
zoisite
Conoscopic lightConoscopic light
Objectivelens
Mineral
Condensor
Divergent light through the condensor
Light cone (±30°)Light cone (±30°)
WPolariser
N-S Analysator What do we see?What do we see?
n
n
n
n
nn
nn
Various slices through the indicatrixVarious slices through the indicatrix
INTERFERENCE FIGUREINTERFERENCE FIGUREE
Conoscopic ray pathsConoscopic ray paths
Retardation () is NOT constant! n dependent on angle Different interference Different interference
colourscolours
Uniaxial interference figureUniaxial interference figure
Fig. 7-14Fig. 7-14
O E
Colour rings showing interference colours = ISOCHROMESISOCHROMES
Crossing lines that remain in extinction = ISOGYRESISOGYRES
Intersection of isogyres = MELATOPEMELATOPE = the OPTIC AXIS (c)
Sectors between the isogyres = QUADRANTSQUADRANTS
Interference colours increase with distance from the melatope (c-axis)II
IIIIIIIIII
IVIV
Measuring the optical character (+)Measuring the optical character (+)
(+) Mineral with gypsum plate:
NE & SW: ne’ || n Addition
NW & SE: no || n Subtraction
Isochromes in I. and III. quadrants are higher by one order
Isochromes in II. and IV. quadrants are lower by one order
Isogyres red-violet (1º red)
Close to the isogyre ( ≈ 100 nm) 2° blue (650 nm) in NE & SW (100+550) 1° yellow (450 nm) in NW & SE (100-
550)
(+) (+) Mineral:ne' > no
nnee slow ray slow ray
ne’
no
AddAddAddAddSubSub
SubSub
nn
no
ne’
ne’
ne’
no
no
Measuring the optical character (-)Measuring the optical character (-)
(+) (+) Mineral:ne' < no
nnee fast ray fast ray
ne’
no
SubSubSubSubAddAdd
AddAdd
nn
no
ne’
ne’
ne’
no
no
(-) Mineral with gypsum plate:
NE & SW: ne’ || n Subtraction
NW & SE: no || n Addition
Isochromes in I. and III. quadrants are lower by one order
Isochromes in II. and IV. quadrants are higher by one order
Isogyres red-violet (1º red)
Close to the isogyre ( ≈ 100 nm) 1° yellow (450 nm) in NE & SW (100-
550) 2° blue (650 nm) in NW & SE (100+550)
Uniaxial Optic Axial Figures (OAF)Uniaxial Optic Axial Figures (OAF)
without gypsum plate:same for (+) and (-)
(+) with gypsum plateblue in I. quadrant
(-) with gypsm plateyellow in I. quadrant
OAF with uncentred melatope (Z)OAF with uncentred melatope (Z)
Konoskopische Bilder optisch einachsiger Kristalle bei unterschiedlicher Schnittlage; Isochromaten sind in Grautönen dargestellt. In der oberen Reihe sind Schnittlagen fast senkrecht zur optischen Achse skizziert, in der unteren Reihe deutlich schräger zur optischen Achse, so dass das Isogyrenkreuz außerhalb des Gesichtsfeldes liegt.
How do we get an OAF?How do we get an OAF?
1. In XN, find a grain that remains in extinction through 360º - centre it
2. Change to high-powered objective and focus
3. Make sure grain stays in field of view4. Maximise light (open diaphragm,
remove sub-stage lens)5. Remove left ocular6. You should see an interference figure -
draw it7. Insert gypsum plate and note optic sign